1. Field of the Invention
This invention relates to transfer paper to be used in full-color or monochromatic copying machines and printers using an indirect dry electrophotography and a process for producing the same.
2. Description of the Related Art
High definition of electrophotographic systems has been studied in cope with the development of color copying machines and printers, and digitalization of these systems. In particular, digitalization of input and output information has advanced for obtaining a high quality image with a full-color electrophotographic copying machine or printer and brought about great improvements in imaging (input), image processing, development, transfer, fixing, and the like. Developers and photoreceptors have also been improved in conformity with the progress of digitalization, high definition, and high color development recording.
However, conventional transfer paper which has been used in electrophotographic monochromatic copying machines and printers is unsatisfactory for use in the thus advanced electrophotographic full-color copying machines or printers. That is, the color image formed on the conventional transfer paper has poor graininess in the halftone which is often contained in photographs, suffers from fine unevenness of gloss or density over the halftone to solid area, and also suffers from mottles (density unevenness appearing in specks) in the halftone to solid area.
In order to improve image-forming properties of coated paper for use in indirect dry electrophotographic recording, smoothing of coated paper so as to give controlled air permeability or addition of a non-film-forming resin to a coating layer so as to prevent such image defects as blisters has been proposed (see JP-A-62-198876, and JP-A-3-294600, the term "JP-A" as used herein means an "Unexamined Japanese Patent Publication"). For the purpose of preventing transfer deficiency under a high humidity condition, it has been proposed to control the surface resistivity of coated paper under a high humidity condition above a given level (see JP-A-62-198877) or to use a special emulsion type adhesive in the coating layer (see JP-A-3-242654). Further, coated paper providing a high quality image with high gloss has been proposed, in which specific calcium carbonate is used as a pigment in the coating layer, the coating layer has as small a solids content as 3.5 g/m.sup.2 or less, and the coated surface has a smoothness above a given level by smoothing treatment (see JP-A-4-291351).
However, any of the above-cited publications makes no mention of improvement on graininess in the halftone. It turned out that the toner transferred to the coated paper according to these techniques is extended or spread by the thermal fixing roll. In the case of the coated paper having a small coating thickness, the toner image also tends to be extended by the thermal fixing roll and, in addition, the fibers exposed on the coating layer because of the small coating thickness disturb the image to reduce the graininess.
Also with respect to non-coated paper, various proposals have been made. For examples, it has been proposed to increase smoothness in order to accomplish high image quality (see JP-A-3-131760), to reduce the CSF (Canadian standard freeness) of the pulp of the skin layer in order to improve image quality and dimensional stability (see JP-A-3-180599), or to incorporate a certain kind of polyester particles into the stock in order to improve transfer properties at high humidity (see JP-A-3-186855).
Notwithstanding these manipulations, fibers on the surface of non-coated paper still exert adverse influences on image quality. That is, any of these proposals is not so effective on improvement of graininess in the middle tone of the transferred image or on elimination of fine unevenness of gloss or density in the halftone to solid area of the image.
Mottling or graininess in halftone to solid image areas has been found by the present inventors for the first time to be an image defect which occurs when an image composed of halftone to solid areas for the most part, such as a photographic image, is electrophotographically printed on conventional transfer paper by use of the aforesaid advanced full-color copying machine or printer. This image defect differs from any of the conventionally indicated problems, such as transfer deficiency at high humidity (see JP-A-62-198877, JP-A-5-53363), reduction in density at low humidity and the image defect due to scattering of a toner (JP-A-5-53363), and transfer deficiency due to surface roughness of transfer paper (JP-A-4-291351). There is no report of the problem of mottles still less proposal of means for preventing mottles.